If someone has an old bottle they aren't worried about destroying, I think the heated rock boiling method it is worth a try. A thin film of water/steam between the rock and the plastic may be enough to prevent it from melting. OR, put a layer of pebbles on the bottom to support the hot rock.

As for "toxic chemicals" in everyday items, this is quite OT, but:
Excerpted and heavily snipped from http://www.junkscience.com/foxnews/fn081800.htm
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As I was enjoying some Ben & Jerry’s ice cream at one of their “scoop shops” last summer, I noticed a Ben & Jerry’s marketing brochure titled “Our Thoughts on Dioxin.” The brochure stated, “Dioxin is known to cause cancer, genetic and reproductive defects and learning disabilities... The only safe level of dioxin exposure is no exposure at all.”
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We measured the level of dioxin in a sample of Ben & Jerry’s “World’s Best Vanilla” ice cream.
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Two independent laboratories using different methodologies reported a single serving of the ice cream contained about 200 times the level of dioxin the EPA says is safe
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The story gets better.
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...based on our testing, a single serving of Ben & Jerry’s contains about 2,285 times more dioxin than an 8-ounce “serving” of gasoline refinery wastewater at the permitted level.
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